Bond pads defined in what would normally be the top-most metal layer deposited according to the process flow, provide sufficiently large areas onto which a bonded termination of a connection wire, the size of which is many times larger than the integrated circuit wiring defined in the metal layer, may be made. Bonding is commonly done with a thermo-sonic bonding technique, which is well known. Thicknesses of the normally multiple metal layers, interleaved with isolation layers, of present technology integrated circuit devices are extremely small, and the bonding process may induce microcracks in the metal layer of a bond pad. In time, these microcracks may progressively propagate for many reasons, including the repeated thermal cycling to which the packaged device may be subject during its operating life. Bond pad cratering in another possible legacy of the wire bonding process may subtly lead to failures of the electrical continuity with the surface wiring defined in the metal layer that connects the circuital node of the pad to be accessible through a device pin.
These reliability issues of bond pad connections have fostered research in failure mechanisms and in quality testing techniques. These reliability issues of bond pad connections have also fostered research in monitoring, in time and in a non-destructive manner, the condition of a bond pad to detect signs of a likelihood to fail.
U.S. Pat. No. 6,395,568 discloses a method and apparatus for sensing the existence of bond pad cratering based on the exploitation of a transistor structure. The transistor structure normally underlies the bond pad as part of an electrostatic discharge (ESD) device that is formed under the strings of perimetrically organized bond pads for protecting sensitive circuitry from disruptive electrostatic voltages that may be applied through the device pins while handing them. The method is based on sensing an anomalous current leakage through the ESD structure.
U.S. Patent Application Publication No. 2008/0246491 discloses a method and related system for testing the presence of a crack in a bond pad of a device under test. The device under test is connected to an external test apparatus that includes a bridge circuit coupled to an electrical signal source adapted to measure an impedance between a purposely made test bond pad and a bond pad of the functional circuitry of the integrated circuit to be tested.
U.S. Application Publication No. 2004/0217487 discloses a crack detection structure for a bond pad. The detection structure comprises a stack of a plurality of interleaved electrically conductive layers and non-electrically conductive (isolation) layers. A crack in at least one of the non-electrically conductive layers of the test structure that, differently from the others, has no vias formed therethrough, is detectable as a leakage current between the bond pad and an underlying electrically conductive layer of the structure.
Known bond pad monitoring apparatuses are either complex or generally require numerous stacked layers purposely made under the bond pad or an ancillary test bond pad to be coupled to external test apparatus.